For CO2 specifically, one offline option is the "CO2Meter RAD-0301", also available as "TFA Dostmann AirCO2ntrol Mini" and possibly other labels. They have an unofficial USB interface too: https://news.ycombinator.com/item?id=22764488

Open your windows at least a few inches. Let some breeze through. Don't worry about the AQI number :)

IQAir AirVisual Pro works offline. It's expensive though.